Polish patents No 124110 and No. 125769 disclose methods for the processing of peat that lead to the separation of biologically active substances by leaching an air-dried peat with dried aqueous alkaline solutions. Due to the high adsorbing capacity of peat, however the leaching operation is extremely difficult to carry out on an industrial scale.
The extraction of large quantities of peat in a static way proved to be unsatisfactory. Static extraction is commonly employed to obtain--from crude products having a loose structure and a rather low degree of comminution--extracts that can easily be washed out in batches by means of a suitable extracting solvent. Accordingly, coarsely ground material is flooded with solvent. The extracting fluid is kept in contact with the material for a time sufficient to obtain a saturated solution of the desired substance or substances in the extracting medium. Subsequently, the extract is collected from the bottom of the extractor so that the whole batch of the material is soaked through with the extract. When such a method is applied to a raw, air-dried peat, collection of the extract from the bottom of the extractor is impossible due to the formation of an impermeable mud layer at the bottom parts of the extractor as a result of the sedimentation of swollen peat particles.
For similar reasons a method of soaking peat particles in extracting tubs filled with the extracting solvents, periodical stirring of the mixture and decanting the obtained extract is equally unsuccessful. In such conditions, penetration of the peat with the solvent is very low because the longer the time of contact of the solvent with the peat, the thicker is the impermeable mud layer formed at the bottom part of the extraction tub. Decantation leads to a relatively clear extract, but the concentration of desired substances in such extracts is comparatively low.
More intensive stirring of the mixture increases the penetration of extracting solvent through the peat bed but results in dispersion of the impermeable mud layer and makes impossible the separation of the clear extract by decantation. Sedimentation and swelling of the peat particles cause further problems at the discharge of the extraction tubs and in cleaning them before the next operation cycle. The situation does not improve whether the extracting tubs were filled first with the peat and then the solvent was introduced or when the sequence was reversed, i.e. solvent was poured in first followed by the addition of peat. Furthermore, the desired active substances diffuse into the extracting solvent from the upper parts of the peat bed only, resulting in a relatively low extraction efficiency. Desired substances present in deeper layers of the peat bed are not dissolved but are discharged with the remaining peat after extraction.
It is an object of the present invention to provide an effective and convenient process for extracting peat and obtaining clear extracts which are both free from any solid particles and rich in the extracted substances. This object is achieved by the measures described below. A further object of the invention is to provide a suitable apparatus for carrying out the extraction process. Further improvements of the invention are achieved by the measures also described below.